Burner control systems are designed both to monitor the existence of flame in a supervised combustion chamber and to time and verify a sequence of operations of burner controls and safety interlocks. The safety of the burner operation is a prime consideration in the design of burner control systems. For example, if fuel is introduced into the combustion chamber and ignition does not take place within a reasonable time, an explosive concentration of fuel may accumulate. A burner control system should reliably monitor the existence of flame in the combustion chamber, accurately time a trial-for-ignition interval, inhibit ignition if a false flame signal is present, and shut down the burner in a safe condition whenever a potentially dangerous condition exists. Examples of such burner control systems are shown in U.S. Pat. Nos. 3,840,322 and 4,137,035, and U.S. Pat. Application Ser. No. 9,307 filed on Feb. 5, 1979 by Phillip J. Cade, now U.S. Pat. No. 4,243,372.
In certain furnace systems, the amount of heat provided by the furnace may be continuously varied by means of a modulator which controls the amount of air flowing through the furnace and the amount of fuel which is provided to the furnace burner. Typically, such a modulator includes a motor which simultaneously drives a fuel valve and an air valve to provide such control. In starting up such a burner system including a modulator, it is desirous to provide different air and fuel flow settings during different parts of the start-up procedure for reasons of safety and efficiency. As part of this start-up procedure, certain interlocks should also be checked to determine that the furnace is in a safe condition to begin operation.